Demodulators are used in television applications in order to demodulate the intermediate frequency (IF) output of a tuner down to baseband for further processing by a receiver. Professional demodulators are special examples of demodulators that are typically used in a laboratory or on a production line in order to test modulators, transmitters, and translators. However, professional demodulators have other uses such as monitoring broadcasts or cable transmissions, testing receivers, field testing television transmissions, and as exhibits used in demonstrations and seminars. In such cases, the professional demodulator receives an input signal to be monitored and provides various outputs representing the input signal which is received over a channel.
When testing transmission equipment, for example, there are generally two sources of distortion in the output of a professional demodulator. One source of distortion is the transmission equipment itself. The professional demodulator is typically provided with distortion correcting components, such as equalizers, in order to reduce distortion caused by the transmission equipment being tested.
The other source of distortion is the professional demodulator itself. This type of distortion is generally caused by variations in component performance characteristics and by component performance characteristics which change over time. For example, the tuner of a professional demodulator contributes to such demodulator caused distortion. In the case of tuner caused distortion, not only does the performance characteristics of the tuner change over time, but the performance characteristics of the tuner also change from channel to channel.
It is standard practice to calibrate a professional demodulator in the factory in order to reduce demodulator caused distortion. However, such calibration, although satisfactory at the time the professional demodulator leaves the factory, can soon become unsatisfactory because of the time related changes of the demodulator's performance characteristics.
Moreover, calibrating the professional demodulator in the factory poses the additional problem of determining the channel to which the professional demodulator should be calibrated. That is, a professional demodulator that is optimally calibrated in the factory at one channel is not optimally calibrated at another channel because, as discussed above, the performance characteristics of the demodulator's tuner change from channel to channel. Thus, a professional demodulator that is optimally calibrated in the factory at one channel may not be optimally calibrated when being used to test equipment operating at a different channel.
The present invention is directed to a reference generator which may be used to supply a substantially distortion free ATSC compliant VSB reference signal to a digital demodulator so that the digital demodulator can be calibrated in the field. However, it should be understood that the reference generator of the present invention may be used for purposes other than calibrating a digital demodulator.